1. Field of the Invention
The present invention relates to a printed wiring board, and in particular, to a printed wiring board used in electronic equipment such as information equipment or the like.
2. Description of the Related Art
It is thought that electromagnetic noise, which is radiated from various types of electronic equipment such as information equipment or the like and which has been a problem in the conventional art, is mainly caused by clock signals on a printed wiring board, or by the signal wires of the digital signals which are synchronous with the clock signals. Various countermeasures for preventing electromagnetic radiation have been applied to the signal wires on a printed wiring board, the wire harnesses connected to the signal wires, and the like.
Countermeasures such as, for example, decreasing the slopes of the rise and fall of an output signal by adding a damping resistor or a filter to the signal output wire, or disposing a guard pattern of the ground electric potential in a vicinity of the signal wire so as to make the feedback current loop smaller, or the like, are generally carried out.
Further, there are cases in which an electromagnetic wave radiated from a printed wiring board has a frequency distribution which is different from that of an electromagnetic wave for which it can be predicted, from the current distribution on the signal wire, that the electromagnetic wave will be generated, and has a sharp peak at a specific frequency regardless of the properties of the signal wire. The main cause of generation of such an electromagnetic wave is known to be, as disclosed in Japanese Patent No. 3036629, not the signal wire of the printed wiring board, but the power source system, and is the electric resonance generated at the power layer and ground layer which oppose one another.
Thus, Japanese Patent No. 3036629 discloses a technique in which, in order to lower the reflectance of the resonance current at the end portions of a printed wiring board, a plurality of first capacitors are disposed at the end portions of the printed wiring board. Second capacitors, which are for suppressing the loop current which flows between the first capacitors and active elements such as ICs mounted on the printed wiring board, are connected to the power source terminals of the active elements, or are connected between the ground layer and the power layer in the vicinity of the power source terminal.
However, in the technique disclosed in Japanese Patent No. 3036629, due to the inductance caused by the capacitors themselves and by mounting the capacitors, the aforementioned effect is not achieved with respect to resonance current of high frequencies exceeding, for example, about 1 GHz. Further problems arise in that, at low frequency bands as well, it is not possible to completely eliminate the resonance current, and electromagnetic waves radiated from the end portions of the printed wiring board cannot be completely suppressed.
Moreover, the source of radiation of the electromagnetic waves caused by the power source system of the printed wiring board is the current which flows through the power source wires of a digital IC mounted on the printed wiring board, at the time of driving the digital IC. Thus, in the technique disclosed in Japanese Patent No. 2734447, to achieve the object of isolating the power source surface of the printed wiring board and the digital IC at high frequencies, in order to increase the high frequency impedance, branch wires, which are in zigzag forms or crossing forms or the like, are formed, and further, the insulating materials of both upper and lower sides of the power source surface are formed from a material which includes a magnetic body. In this way, the high frequency power source current which flows into the power source surface can be reduced.
Japanese Patent Application Publication (JP-B) No. 7-46748 discloses the following technique, which has similar effects as those of the technique disclosed in Japanese Patent No. 2734447. Namely, JP-B No. 7-46748 discloses providing a secondary power source surface, which is physically isolated from the main power source surface, at the printed wiring board. The main power source surface and the secondary power source surface are connected via a filter. The secondary power source surface is decoupled from the ground surface by a plurality of capacitors, and supplies power.
However, in the same way as the technique disclosed in Japanese Patent No. 3036629, with the techniques disclosed in Japanese Patent No. 2734447 and JP-B No. 7-46748, it is not possible to completely suppress the high frequency current flowing between the power source surface and the ground surface, nor is it possible to completely suppress the electromagnetic waves radiated from the end portions of the printed wiring board. Further, by providing the capacitors for decoupling at the secondary power source surface, the impedance between the main power source surface and the ground surface, which is a cause of electromagnetic wave radiation, conversely becomes greater. Thus, other capacitors must be provided at the main power source surface. Secondary problems thus arise in that the number of parts increases, and the stability of the electric potential at the ground surface deteriorates.
Namely, because many capacitors which couple the power source surface and the ground surface are added, a large number of opening portions (clearances), which are provided at the ground surface in order to allow passage of via holes which connect the capacitor pads and the power source surface, must be provided. It would be ideal for there to be no electric potential differences at the respective regions on the ground layer. However, because a plurality of opening portions are provided adjacent to one another, inductance is generated at the regions between the opening portions, and an electric potential difference arises at the both ends thereof. Moreover, because the metal region of the ground surface becomes smaller due to the provision of the many opening portions, the stability of the electric potential of the entire ground surface deteriorates.
Moreover, electromagnetic wave radiation of the printed wiring board due to common mode noise caused by the return current of the current flowing through the signal wire being incomplete, and radiation noise caused by the loop current, are also problematic.
Japanese Patent Application Laid-Open (JP-A) No. 11-233951 discloses a technique in which, in order to prevent the return current path of the current flowing through the signal wire from being cut-off due to the power layer being divided into the main power source surface and the secondary power source surface, the opposing power layer and ground layer are connected via capacitors, and the high-frequency return current is bypassed to the ground layer.
However, in this technique disclosed in JP-A No. 11-233951, the return current is bypassed by connecting, at high frequencies and by using two capacitors, the main power source surface and the secondary power source surface which are disposed at the same layer. Thus, the inductance component of the capacitors themselves, and the inductance component of the via holes for connecting the capacitors between the power layer and the ground layer or of the pads for connecting these via holes and the wires, are introduced in series into the return current path. Thus, the inductance cannot be made to be sufficiently low in the high frequency region of, for example, several GHz or more.
Moreover, JP-A No. 11-330703 discloses the following technique in a printed wiring board which is formed by a plurality of ground layers being disposed between a plurality of mixed layers in which signal wires and power source wires exist in the same layer. The plurality of ground layers are connected via a plurality of via holes, and the return current path at the time when current flows at the plurality of mixed layers is continuous. In this way, the loop formed by the return current is made smaller, and the electromagnetic wave radiation is thereby suppressed.
However, in this technique disclosed in JP-A No. 11-330703, the signal wires and the power source wires are disposed in the same layer. Thus, there are the problems that the degrees of freedom in design markedly decrease, and it is difficult to arrange, at a high density, the wires or the parts which operate at high speed which have come to be greatly desired in recent years. Moreover, in recent years, capacitors for decoupling or the like have been disposed in the vicinity of the device which is the source of the electromagnetic wave radiation when power is supplied by the wires, such that the transient current having the high frequency component is closed-in. Further, generally, a filter is provided so that the high frequency current cannot flow on the power source wires, and there is no need to consider the return current on the ground layer.
As described above, in a printed wiring board in which the power layer and the ground layer oppose one another and the surface area over which they oppose one another is large, electromagnetic wave radiation from the end portions of the board, which is caused by the resonance current flowing substantially symmetrically to the power layer and the ground layer, is dominant. However, for resonance current of high frequencies such as frequencies exceeding 1 GHz, the electromagnetic wave radiation from the end portions is not dominant. For resonance current of lower frequencies as well, electromagnetic wave radiation cannot be completely suppressed. On the other hand, the working speed of digital circuits in recent years has increased more and more, and frequencies have become higher and higher.
Further, in order to transmit high frequency signals on a printed wiring board, it is essential to lower the impedance of the signal wires, and to stabilize the electric potential on the ground source surface and the power source surface where the return current for the signal wires flows.
In a printed wiring board of a structure in which the power layer and the ground layer oppose one another and are disposed at the internal layers, in order to reduce the impedance of the signal wires, either the distance between the power layer and the ground layer must be made large, or plural power layers and ground layers must be provided. Problems arise in that, in the former case, the electromagnetic wave radiation due to the resonance current increases, and in the latter case, the costs increase.
In recent years, demands to increase the speed of circuits and to decrease the size of printed wiring boards have increased. Wire layers, in which the signal wires are wired, are completely filled up by the signal wires such as the bus lines and the like. Thus, in a case in which, in order to prevent electromagnetic wave radiation due to resonance current, wires for supplying power to the wire layers are provided without forming a power layer at the inner layers of the printed board, a problem arises in that the density of mounting decreases.